Bitcoin is gonna be the (or at least a major) currency of the future, and I'd like to gather as much as I can now that they're still cheap.
However, don't get me wrong, this is by no means a reason not to spend bitcoins right now!
You see, pretty much any place that accepts bitcoins nowadays, still uses the current USD or EUR rates to determine their bitcoin pricing. Well, instead of buying their products with euros or dollars, I'd rather buy bitcoins and then pay for the products with bitcoins. Costs about the same (sometimes even cheaper due to USD/EUR differences) and helps boosting the bitcoin economy.
So I'm only using exchanges one way: from euros/dollars to BTC.